Gin wins. The fact is, Gin was better than the average Captain back then. In fact, he was probably the most skilled captain if we discount the Elder Captains (Unohana, Yama, Shunsui, and Ukitake), Aizen, and probably Kenpachi, all of whom would beat Candice. Gin just needs to get just one shot - one scratch - in, and disintegrate Candice. Candice, on the other hand, just doesn't have much of a chance. She'll need to hit him with several attacks and more before he goes down. Gin's Bankai is not slow and he'll be able to tag her before she's able to beat him down.
And considering Byakuya could easily beat these kind of opponents, I doubt Gin would have much trouble.